Certain lipid-filled cavities that form within the wall of a blood vessel are known as “vulnerable plaques.” These plaques, when ruptured, can cause massive clotting in the vessel. The resultant clot can interfere with blood flow to the brain, resulting in a stroke, or with blood flow to the coronary vessels, resulting in a heart attack.
To locate vulnerable plaques, one inserts a catheter through the lumen of the vessel. The catheter includes a delivery fiber for carrying infrared light that will ultimately illuminate a spot on the vessel wall and a collection fiber for carrying infrared light scattered from a collection area on the vessel wall.
The distal tip of such a catheter includes a stationary transparent jacket enclosing a rotatable housing that holds the delivery and collection fibers. In addition to these fibers, the housing encloses two mirrors: one to bend a beam exiting the delivery fiber so that it illuminates the wall; and another to gather scattered light from the wall and to direct that scattered light into the collection fiber.
A vulnerable plaque can be anywhere within the wall of the vessel. As a result, it is desirable to circumferentially scan the illuminated spot and the collection area around the vessel wall. One way to do this is to spin the multi-channel catheter about its axis. This requires providing a torque cable and coupling the housing to the torque cable.
The housing at the distal tip, which is already crowded with optical elements, must now accommodate a coupling element to enable torque transmitted by the torque cable to rotate the housing. One way to accommodate this additional element is to enlarge the housing. However, an enlarged housing at the distal tip of a catheter is undesirable because of the limited size of the blood vessels through which the catheter is intended to pass.